There isn't really any vagueness about when to use must and when to use should.
Must always implies absolute obligation or certainty.
Should always implies a request, suggestion, or expectation although in some contexts a request may be so strong that it could be seen as an obligation.
See @Cerberus's excellent answer here exploring the "built-in slipperiness" of English words commonly used in the general area of volition/expectation - where must applies to both at the extremes of obligation/certainty. Things only really get murky at the lesser levels.
But OP's example 2 is structurally ambiguous - You must cross the street to get to that store could mean any of...
You are obliged/I order you to cross the street [and thereby to get to that store]
You would/will have to cross the street if you wanted/need to get to that store
More naturally we interpret the whole sentence as informational, with an implied if you want to get to the other side. The obligation implied by must doesn't come from the speaker - it comes from the laws of physics which say the only way to be on the other side of the street is to cross it.
Note that in practice, people often deliberately or unwittingly flout these distinctions. I'm sure the vast majority of people who have ever said, for example, "I must be mad!", or "We must have dinner together soon" didn't really mean they were absolutely sure, or imposing an absolute order.
Coming is from the viewpoint of the destination.
You might say to your colleagues "I'm going home" then phone your partner and, if they are already there, say "I'm coming home".
While you are home you might say "I came home", though you could also say "I went home" positioning your speech relative to where you were at the time you set out on your journey.
In the imperative, you can ask someone to come to where you currently are, "please come home". You can use either come in terms of where you will also be later, "come to the club with me", though you can also use go to ask them to join you in your act of going, "go to the club with me".
Best Answer
In US usage, joiner would not be used to describe a new employee. Both of the definitions you cite are common useage, but the latter is reserved for those who are prone to join many activities, not just someone who recently joined one activity or a company.
Joinee is not used in the US.
Terms like new employee, new recruit, latest member, can be used. Terms like tyro, rookie, novice, fledgeling, newbie, newcomer, entrant, freshman, neophyte, and starter might be used if you wish to emphasize the newness of the person.